The present invention relates to a new and improved front wall and support arrangement at the headbox of a papermaking machine.
Generally speaking, the front wall and support arrangement of the present invention comprises a relatively stiff support structure supported by a support carrier of about the same length as the support structure. This support structure extends over the entire machine width transverse to the direction of travel of the paper web and is positioned above the slice chamber of the headbox.
The front wall at the headbox of a papermaking machine is an important element as concerns the shaping or configuration of a proper lip geometry, that is to say, the distance between a lower lip and an upper lip of the headbox. This exact configuration of the lip geometry is responsible for the quality features or characteristics of the produced paper web. This precision is especially important with wide and high-speed operating papermaking machines. It is known that deviations in the lip geometry can lead to up to 10 to 15 times greater deviations of the weight per unit area or surface weight profile of the paper web.
At the upper lip a diaphragm or baffle plate defines the distance between the upper lip and the lower lip and thus the size of the slice opening or outlet. This diaphragm or baffle plate and the adjusting devices for the exact adjustment thereof are carried by the front wall.
The front wall itself, nowadays built as an upright box support or girder, constitutes the upper boundary of the nozzle or slice chamber of the headbox. Therefore, this box support or girder is subjected to the pressure of the warm stock slurry or stock suspension which exits through the slice opening or outlet. The pressure and the temperature in the slice chamber as well as the weight of the front wall itself lead to deflections which may be either concave or convex in respect to the slice chamber. It is therefore endeavored to counteract the deforming effects or influences exerted upon the box support or girder in the reverse direction in order to compensate for the deformation and to bring it towards zero level, i.e. to eliminate the same as much as possible. This is done so as to achieve as uniform as possible or as straight as possible cross-section or profile of the slice opening or outlet or the height of the slice chamber, respectively, and as a result as good as possible uniformity, both locally and as a function of time, of the flow of the stock through the elongated slice opening or outlet.
A known front wall construction utilizes an additional support carrier which extends along and above the front wall. To counter the deformation forces there is provided a chamber between the support carrier and the front wall. This chamber contains a pressure medium or heating medium.
This state-of-the-art front wall has proven to be quite satisfactory. However, the total or overall constructional height is very large, about twice the height of the front wall itself. Sealing problems pose further difficulties during sealing of the pressure chamber.